It is known to use a hydraulic seal to seal between two relatively rotating shafts. U.S. Pat. No. 6,568,688 describes such a seal, in which oil is continuously fed from the low-pressure side of the seal, passes through the seal to provide cooling, and is recovered to the low-pressure side of the seal for scavenging.
FIG. 1 is a sectional view, at top dead centre, of a known hydraulic seal arrangement, as described in U.S. Pat. No. 6,568,688. A low pressure shaft 12 and a high pressure shaft 14 are concentric shafts of a two-shaft gas turbine engine, rotating at different speed about a common axis of rotation 16. The high pressure shaft 14 is supported on a bearing 18.
To the right-hand side of bearing 18, and radially outward of high pressure shaft 14, is a region 20 of relatively low pressure; and to the left-hand side of bearing 18, and radially inward of high pressure shaft 14, is a region 22 of relatively high pressure. The two regions and 22 must be effectively sealed from each other in operation, and to achieve this a hydraulic seal, indicated generally by 24, is provided between the two shafts 12 and 14.
The hydraulic seal 24 comprises an annulus 26 which is provided in the interior of the high pressure shaft 14 and extends radially outwards across the whole circumference of the shaft 14. The annulus is defined by radially inwardly extending walls 27, which extend around the whole circumference of the shaft 14. Projecting into the annulus 26 is a web 28 which is arranged on the low pressure shaft and extends radially outward across the whole circumference of the shaft 12.
In operation, a large portion of the annulus 26, and especially that portion of it that surrounds the free end of the web 28, is filled with oil or some other hydraulic medium 30, as indicated by the hatched area.
In operation, a continuous feed of oil is maintained into the annulus 26, in the following manner. An oil jet (not shown) between the two shafts 12, 14 delivers oil in the direction shown by the arrow 32. Some of this oil serves to lubricate the bearing 18; the remainder tends to collect on the inner wall 34 of the high pressure shaft 14, owing to the centrifugal effects arising out of the rotation of the shafts 12, 14. The inner wall 34 lies at the radially outward side of the space between shafts 12 and 14.
It is also possible, in an alternative embodiment of the known arrangement, for the oil to be fed in along the centre of the hollow shaft 12.
Again under centrifugal effect, oil also enters the annulus 26 through an annular inlet area 36, the annulus 26 lying still further radially outward than the inner wall 34. In the process, the oil 30 collects in the annulus 26 both to the left and to the right sides of the web 28, creating an optimum siphon-type hydraulic seal as shown.
It will be appreciated that the radially inwardly extending side walls defining the annulus 26 act as weirs and limit the oil level within the annulus. The surface or liquid level of the oil, on each side of the web 28, will be ultimately constrained by the radially inward extension of the respective side wall. This can be seen in the annular inlet area 36.
On the left-hand side of the web 28, the surface or liquid level 38 of the oil 30 is further radially outward than on the right-hand side of the web 28, because the left-hand side communicates with the region 22, in which the pressure is higher than in the area 20 with which the right-hand side is in communication.
A scoop plate 40 in the annulus 26 defines a passageway 42, through which excess oil can flow to an outlet duct 44. In this way, a continuous flow of oil is maintained through the hydraulic seal 24. (Other ways of achieving throughflow are known, and will be familiar to the skilled reader of this specification; for example, oil may flow over one of the weirs, typically on the opposite side of the web 28 to the oil jet.) This prevents undesirable overheating and coking of the oil 30, which would occur if the oil was allowed to remain for too long within the annulus 26. In general, the amount of heat transferred to the oil is highly dependent on the depth of immersion of the fin 28 in operation.